Embodiments of the present invention generally relate to wavefront (phase gradient) detecting devices. More specifically, embodiments relate to a light-field pixel for detecting a wavefront. The light-field pixel includes an aperture layer covering a light detector layer (e.g., layer having a complementary metal-oxide-semiconductor (CMOS) imaging sensor array). The aperture layer includes a conventional aperture and a non-conventional aperture. The non-conventional aperture is designed to generate a higher gradient of transmission at normal incidence than the conventional aperture. A conventional aperture has a gradient of transmission close to zero at normal incidence. To isolate the phase gradient from the amplitude information, the intensity of light through the non-conventional aperture is normalized by the intensity of light through the conventional aperture.
An optical microscopy image contains two major types of image information: light amplitude and optical phase spatial distribution. Amplitude image information is readily extractable as optical detectors, ranging from our retina layers to a CCD camera, are sensitive to light intensity—the square of the light amplitude. From the light amplitude variations, we can discern scattering structures, fluorophore sites, absorption sites and other relevant structures in the sample. Acquiring light amplitude spatial distribution is the primary function of a conventional microscope.
Unlike the light amplitude distribution, the optical phase distribution associated with a microscope image is more difficult to extract. Generally, optical phase detection requires the use of interferometry to encode the phase into amplitude variations. This entails the use of more elaborate optical arrangements. Phase information is useful as the optical phase delay is a sensitive measure of refractive index variations. As an example, a phase sensitivity of 5 degrees at wavelength 600 nm translates to an ability to discern a refractive index variation of 10-3 in a 10 micron thick sample.
In the biomedicine setting, two phase microscopy methods dominate: phase contrast microscopy and DIC microscopy. The phase information that each provide is different. Phase contrast microscope tends to highlight locations of high scatter—it derives contrast by interfering scattered light components with unscattered light components. On the other hand, DIC microscope tends to highlight regions where the refractive index of the sample is rapidly changing. Both techniques can be adapted into a conventional microscopy setup. However, the requisite optical arrangements are elaborate and, as such, phase microscopes are expensive and relatively high maintenance. In addition, both techniques share a common shortcoming—in both cases, the phase information is inextricably mixed with the amplitude information. In other words, a dark spot in the acquired phase contrast or DIC image can be due to a corresponding absorptive spot on the sample or a phase variation—there is no way to distinguish the two effects without additional measurements. This shortcoming also prevents phase contrast and DIC from providing quantitative phase measurements.
Besides phase contrast and DIC microscopy, various full field quantitative phase imaging techniques have been recently developed. Some of the prominent techniques are: 1) phase shifting interferometry schemes—where two or more interferograms with different phase shifts are acquired sequentially and a phase image is generated from them, 2) digital holography or Hilbert phase microscopy—where high frequency spatial fringes encoded on the interferogram are demodulated to generate the phase image, 3) Swept-source phase microscopy—where modulation in the interferogram generated by a wavelength sweep can be processed to create a phase image, 4) Polarization quadrature microscopy—where phase images are generated by a polarization based quadrature interferometer, and 5) Harmonically matched grating-based phase microscopy—which makes use of non-trivial phase shifts between the different diffraction orders from a harmonic combination grating to generate phase images. These methods do provide quantitative phase information and have been demonstrated to perform well. However, as with phase contrast and DIC microscopy, most of these advanced methods contain significant optical elements and have relatively steep learning curves. In addition, this class of phase microscopy techniques invariably requires the use of a laser source to provide coherent light. In comparison, phase contrast and DIC microscopes work well with the usual standard microscope light sources.